The "slot 3" HP buff is the same line as Cleric's "Symbol of xx" line. Basically any cast symbol or temp/virt/tena line will overwrite this, or block it form being used. Am not sure if the familiars were intended on having stacking issues intentionally, to make them only situationally useful, or whether it's an error to be fixed after Legends of Norrath Beta is finished.

At this time (at least) this familiar will not take hold if you have Virtue. The buff-blocking ceiling may be lower than that. (Not an issue to me, the hp buff when out places with no buffs is still a nice bonus).

If you are a pet class you cannot have the familiar out and cast your pet. However, when my chanter cast pet first then familiar the icon showed up in the buff window and the benefits remained (as did original pet).